The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A hybrid vehicle generally includes an engine and a motor generator to selectively provide torque to the transmission. The transmission transmits torque to a driveline. The hybrid configuration may be a belt alternator starter (BAS) system. The BAS system is characterized by a combined motor generator used in place of a standard alternator which is connected to the crankshaft of the engine via an accessory drive belt. The motor generator selectively provides positive torque to assist the engine or negative torque which adds a load to the engine. Assisting the engine draws electrical energy from a charge storage module, typically a battery. Adding load to the engine produces electrical energy which may be used to charge a charge storage module or feed the vehicle's electrical loads.
To reduce fuel consumption, the engine may be selectively stopped when the hybrid vehicle comes to a stop. This will be referred to as an auto-stop. During the auto-stop, the vehicle's electrical loads are provided by a charge storage module. The engine may restart when a driver lifts his/her foot off the brake pedal.
The gear selection of an automatic transmission of a hybrid vehicle is controlled by oil pressure in the transmission. The transmission includes a mechanical oil pump that applies appropriate hydraulic pressure for gear shifting. The transmission's mechanical oil pump is directly driven by the engine's crankshaft. When an engine auto-stop is commanded, for example only, when the vehicle stops for a stop light, the mechanical pump is disabled due to lack of power from the engine. An auxiliary transmission oil pump that is driven by an electric motor is enabled during the engine auto-stop. The auxiliary pump supplies oil to the transmission during the auto-stop to maintain a predetermined level of oil pressure in the transmission to maintain clutch pressure. Therefore, when the engine is ready for restart, the transmission is ready to transmit torque to the driveline. The auxiliary pump contributes to a smooth transition of torque to the driveline at engine restart. The auxiliary pump ensures that no undesired clutch slip occurs in the transmission during the engine restart.